A high performance, bias tunable, p-GaAs homojunction interfacial work function internal photoemission far-IR (FIR) detector has been demonstrated. A responsivity of 3.10 +/- 0.05 A/W, a quantum efficiency of 12.5 percent, and a detectivity D* of 5.9 X 1010 cm (root) Hz/W, were obtained at 4.2K, for cutoff wavelengths form 80 to 100 micrometers . The bias dependences of quantum efficiency, detectivity, and cutoff wavelength have been measured and are well explained by the theoretical models. The cutoff wavelength is modeled by a modified high density theory, and the quantum efficiency is predicted by scaling the free carrier absorption coefficient linearly with the doping concentration. The effect of the number of layers on detector performance and the uniformity of the detectors have been discussed. A comparison with Ge:GA photoconductive detectors suggests that a similar or even better performance may be obtainable.